A low voltage current supply distribution network of a vehicle typically operates in the range of 11-16V and supplies electrical energy to consumers in the vehicle, said consumers providing different functions. These functions can be by way of example the control of the drive train and also functions relating to safety, comfort and entertainment. Furthermore, energy is to be provided for a starter battery in order to maintain said battery in a charged sate. In the case of the starter battery of a motor vehicle, said starter battery is by way of example an accumulator that supplies at least the electrical current for the starter of a combustion engine. A starter battery of this type can furthermore support the lighting operation of the vehicle and is usually charged by means of a generator such as the alternator while the vehicle is travelling. The battery of an electric vehicle that is used for driving the vehicle is described in contrast as the traction battery. In addition, electric vehicles or hybrid vehicles can also comprise a starter battery. By way of example, lead acid accumulators or lithium ion accumulators can be used as batteries, however said accumulators are also described hereinunder as lead acid batteries or lithium ion batteries.
The primary supply can be provided in a conventional vehicle by an AC generator or in a hybrid vehicle or solely electric vehicle by a DCDC converter. Since the consumers and the battery are usually connected in parallel, when selecting a desired voltage value for the primary supply, it generally involves a compromise between multiple requirements that are to be fulfilled. By way of example, low power losses can be desired which for vehicles that are driven by fuel greatly reduces the fuel consumption. On the other hand, in the case of electric vehicles and hybrid vehicles, low power losses render it possible to travel a good distance on electrical power. Furthermore, it should be possible to operate the system safely and reliably and for the battery to have a long serviceable life. In addition, the system should be robust, wherein the magnitude of the robustness can be defined as supplying electrical consumers with the required power whilst simultaneously maintaining the defined minimum voltage level.
The operating state of the vehicle can be included in the charging strategy in order to increase the serviceable life of the starter battery. The document U.S. Pat. No. 4,937,528 provides by way of example in an exemplary embodiment that a generator such as an alternator is switched on and off in dependence upon the stage of charge of the battery and the operating state of the vehicle. The generator can be isolated if the state of charge of the battery has achieved a defined value and has not yet dropped back below a lower threshold value. However, if the vehicle is in a situation where the demand for current is increased, it is possible to provide that the generator remains switched on even if the state of charge of the battery has already returned to a defined threshold value. These situations can occur by way of example at night in winter or as the air-conditioning is operating.
The previously mentioned targets that are to be achieved with regard to the construction and also how these targets are to be achieved are important in particular in an electric vehicle or a plug-in hybrid vehicle, since, although vehicles of this type are constructed with a high as possible energy efficiency, the batteries are however charged for long periods of time at an electrical socket. As a result of the loss of water, the long charging periods would lead to premature malfunctions of the battery or rather to the battery becoming damaged if the continuous charging process were to be performed at the electrical socket using a conventional charging voltage. In addition, power losses in the battery and in the vehicle consumers would accumulate in the case of a conventional charging voltage, which over time could lead to a high amount of energy being wasted.